A power amplifier in an RF transmitter operating at high power levels has non-linear amplifier characteristics that can produce undesired intermodulation distortion due to interaction between the signals being amplified. Often referred to as a non-linear device, a power amplifier has linear regions and non-linear regions. To avoid signal distortion, a power amplifier needs to be used in the linear regions.
In the non-linear regions, signals from the power amplifier will be subject to amplitude to amplitude modulation. This is caused by the fact that as the power amplifier is operated in the non-linear regions, the ratios of input to output power are not constant. As the input signal amplitude increases, there will be a disproportionate increase in the output power. This is called amplitude modulation/amplitude modulation (AM/AM) since an unwanted additional amplitude modulation is experienced.
As an example, AM/AM distortion will be experienced up to the maximum output power at which point all input values will have the same output value. This is called compression and will result in the signal being clipped. This will result in the signal having square or sharper edges in the time domain which means that higher frequency components will be generated and side lobe re-growth in the frequency domain will be experienced. This can and often does cause out of band emissions in addition to distorting the amplified signal.
Envelope feedback is often used for correcting distortion in an output signal from a power amplifier. However, traditional envelope feedback's strength or gain depends on the waveform envelope. This makes it difficult to maintain a consistently strong feedback. As waveforms move to higher peak-to-average ratios with higher symbol rates, the envelope gain dependence often results in insufficient distortion correction.
One approach for correcting distortion in an output signal from a power amplifier is disclosed in U.S. Pat. No. 6,711,217. A baseband linearization arrangement receives the baseband signal, demodulates a distorted amplifier output signal, and compares the received baseband signal with the demodulated amplification output signal for providing a predistorted signal to remove the distorted component of the amplifier output signal applied by the power amplifier. A carrier band linearization amplification arrangement amplifies the output signal of the baseband linearization arrangement to linearize the distorted component of the amplifier output signal using the predistorted signal. The distorted component from the output of the power amplifier is linearized by extracting an error signal and amplifying the error signal to be combined with the amplifier output signal to eliminate any distorted components in the amplifier output signal.
Another approach for correcting distortion in an output signal from a power amplifier is disclosed in U.S. Pat. No. 7,215,716. Pre-distortion signals are generated such that when these signals are amplified, the non-linear distortions of the power amplifier are opposite that of the pre-distortion. This is based on using a predistortion look up (LUT).